I know some people might think I'm crazy, but I love the cheapo, Walmart Flat Black. If I put it on real metal, you can polish it with steel wool and get more shine from it, or you can scrub harder with the steel wool and fairly easily expose the metal underneath.
...And I think it is the only paint left you buy for under a buck!
I used testor's gun metal black all the time about ten years ago and it had a glittery undertone that it doesn't seem to have anymore. I bought a jar of it last month and found it to be more blueish and less metallic now. Maybe I just got a bad batch, but I don't think the color as it is now would work that great. They do make something called Testors pearl black I think that I've seen on some blasters and it looks very nice.